This invention relates to an electroless copper plating bath and an electroless copper plating method which can stably provide an electroless copper plating deposit having excellent appearance and physical properties.
Conventionally, for the purpose of preventing any self decomposition of an electroless copper plating bath and forming a dense electroless copper plating deposit having a proper luster by catching minute particles of a catalyst metal developed in the bath and sequestering cuprous ion Cu.sup.+ (I) developed in the reaction, a slight quantity (usually ppm order to 10 ppm order) of a cuprous-ion-complexing agent which forms Cu(I)-halogen, Cu(I)-N, Cu(I)-S complexes or the like is used as a stabilizer.
Among these stabilizers, cyanides are known to have an excellent effect on forming a highly dense electroless copper plating deposit and stabilizing the electroless copper plating bath. In particular, a metal-cyano-complex such as K.sub.2 [Fe(CN).sub.6 ], K.sub.2 [Ni(CN).sub.4 ] or K.sub.3 [Co(CN).sub.6 ] has a wider permissible range of the addition amount to the bath than other stabilizers and the addition of an excess amount of the metal-cyano-complex causes little influence to the deposition rate. That is to say, a metal-cyano-complex, in contrast with other stabilizers used in electroless copper plating, has little possibility of a very small concentration having a great influence on the appearance, the surface condition and the physical properties of the plating deposit as well as the deposition rate and the deposition condition by firmly adhering or adsorbing to the surface of the plating deposit and inhibiting its catalytic activity.
However, a metal-cyano-complex stabilizer has the disadvantage of not lasting long. For example, the stabilizing effect of a metal-cyano-complex stabilizer is lost or greatly reduced during use and even before use if the bath is left to stand. In general, when the stabilizing effect of the stabilizer is lost, a plating reaction can not be suppressed and the plating rate increases, forming a rough deposit and even inducing decomposition of the bath. The stability of the bath can be restored by adding a required quantity of a non-metal-cyano-complex stabilizer. In contrast, in a metal-cyano-complex stabilizer, stoppage of plating reaction is caused when the stabilizing effect is lost and it can not be restored by addition of the metal-cyano-complex, resulting in discarding the electroless copper plating bath at the worst. Due to this the use of a metal-cyano-complex, despite its excellent characteristics as a stabilizer for electroless copper plating, is not practical.